Purification of native or non-native recombinant proteins from bacterial or eukaryotic cells often requires several steps. Methods to reduce the number of steps in a purification of a target protein are advantageous for cheap and efficient production of recombinant proteins. Purification of target proteins fused to small highly basic domains is disclosed by Graslund et al., Protein Eng. 2000, 13(10):703-709, Graslund et al., J Chromatogr A. 2002, 942(1-2):157-166 and Graslund et al., Journal of Biotechnology, 2002, 96: 93-102. These publications describe the rational design of highly basic and stable mutants of the Z-domain from S. aureus protein A to purify different target proteins expressed in E. coli using cation-exchange chromatography. Use of highly basic derivatives of the Z-domain as fusion tags is also disclosed in WO 00/6343.
The present invention provides a new group of positively charged tags which can be used to purify recombinantly expressed proteins to very high purity in few steps.